Storage container



G. D. MORDHORST STORAGE CONTAINER April 16, 1968 Filed Oct. 22, 1965 FIG. 4.

FIG

52 I INVENTOR Gerald D .Mord horst BY WW1 ATTORNEYS A ril 16, 1968 G. D. MORDHORST STORAGE CONTAINER 2 Sheets-Sheet 2 Filed Oct. 22, 1965 FIG.6

ATTORNEYS United States Patent 3,377,813 STORAGE CONTAINER Gerald D. Mordhorst, Boulder, Colo., assignor to Cryogenic Engineering Co., Denver, Colo. Filed Oct. 22, 1965, Ser. No. 502,084 24 Claims. (Cl. 62-45) This invention relates to fluid storage containers and more particularly to a device for improving the evaporation characteristics of such containers.

Cryogenic fluids and the other materials that must be maintained at extremely low temperatures are frequently stored in Dewar type containers. That is, double-walled vessels wherein the space between an inner wall and an outer wall is gas evacuated. In order to prevent any substantial heat leak between the two walls, a variety of bulk insulation materials have been utilized in the gas evacuated space. The state of the art has become so advanced that heat leak into the inner container through these bulk insulation materials has been reduced to the point where further improvements in the bulk insulation between the double walls have almost a negligible effect upon the overall evaporation characteristics of cryogenic storage vessels.

Because of the excellent qualities of the insulation materials that are presently available for use in the vacuum space of Dewar vessels more and more attention is being directed to other methods of reducing evaporation losses. One of these methods, for example, uses the evaporating or boil-off gas itself as a refrigerant to cut down on heat losses.

Boil-off gases are also associated with another major area of heat loss in cryogenic storage vessels. That is, because the boil-off vent structure must extend through the evacuated portion of the vessel and into warmer surroundings it acts as a conduit for heat flow through the evacuated area and into the vessels storage chamber. The entry structure for filling the vessel has similar drawbacks. Moreover, because of the vast improvements in the other areas of heat loss in Dewar vessels, the heat losses through these filling and vent tubes have in many cases become a major portion of the total heat flux into the vessels storage chamber. This is so for two reasons. The first is the purely mathematical proportion reason which needs no explanation. The second, as will be described more fully later, is that the total entry tube heat loss may actually increase as other heat losses are decreased. Hence, although it is a general object of this invention to provide an improved Dewar type storage container, it is a more particular object of the invention to provide a means for substantially reducing the heat losses through such a containers entrance and exit tubes.

One of the major types of entry tube heat losses, particularly in helium Dewars, is caused by thermal acoustical etfects. This type of heat loss has been long recognized and occurs in tubes having one closed end with one end of the tube warm and the other cold. A thusly constructed tube gives rise to the phenomenon of spontaneous oscillations and energy exchanges of the fluid within the tube. This type of heat loss derives its name from the sounds that can be produced by the fluid oscillations which are caused by the thermal gradient across the tube. Lord Rayleigh made reference to this phenomenon in his classic work, The Theory of Sound (vol. 2, p. 230, Macmillan, New York, 1896), where he described it in connection with oscillations of the hot gases in a glass blowers tube. As is recognized in the first edition of Experimental Cryophysics, a Butterworths Ltd. publication edited by Hoare, Jackson, and Kurti (1961), these thermal acoustical oscillations are frequently present in low temperature apparatus. This latter publication also noted that 3,377,813 Patented Apr. 16, 1968 these oscillations always add heat to the low temperature portions of the apparatus and are therefore undesirable In fact, as noted by Wexler in a 1951 article (vol. 22, Review of Scientific Instruments at 282 and 941), the spontaneous oscillations in the tubes of a Dewar type vessel can increase the evaporation rate of the storage, container by a factor of one thousand. Probably the most common of these losses occurs in small diameter tubes such as differential pressure liquid level gage lines which are found on many Dewar vessels.

Consequently, it is a further object of this invention to provide a means for decreasing the heat losses due to thermal acoustical oscillations in the entry tubes of Dewar type vessels.

In addition I have discovered that another cause of heat loss through the entry tubes of cryogenic storage vessels is convective currents which carry heat from the vessels outer shell to its inner shell and thus cause the boil-off of the cryogenic fiuid to increase. Moreover, I have found this type of heat loss in all types of entry tubes such as fill lines and gage lines as well as vent tubes. It is another object of this invention, therefore, to provide a method and apparatus for eliminating or substantially reducing these entry tube heat losses caused by convective currents.

It is a further object of this invention to provide a single structure for substantially reducing the entry tube heat losses caused by both the thermal accoustical effects and the convective currents.

In accordance with the method of the invention the entry tube heat losses in a Dewar type storage vessel are substantially reduced by inhibiting the convective currents of relatively warm fluid which flow from the warm end of the tube towards the cold end thereof.

In accordance with another aspect of the invention a trap is placed in each of the vessels entry tubes so that an intermediate portion of the entry tube is lower than the warm end. In this manner the relatively cold fluid passing out of the entry tube fills the lower portion of the trap and prohibits relatively warm fluids from entering the storage portion of the vessel.

In accordance with still another aspect of the invention therelatively cold fluids from the storage container are forced through a tortuous path as they pass from the vessels inner wall to its outer wall. In this manner, the relatively warm entry fluids are prohibited from entering the storage vessel by the motion of the boil-off gases themselves.

The foregoing and other objects, features and advantages of this invention will be apparent from the following more particular description of preferred embodiments thereof as illustrated in the accompanying drawings wherein the same reference numerals refer to the same parts throughout the various views. The drawings are not necessarily intended to be to scale, but rather are presented so as to illustrate the principles of the invention in clear form.

In the drawings:

FIGURE 1 is a fragmentary elevational view, partially in section, of a Dewar type cryogenic storage vessel, which incorporates a preferred embodiment of the invention;

FIGURE 2 is a developed section of FIGURE 1, partially broken away and taken along the are 2-2;

FIGURE 3 is a fragmentary sectional view of a broken away portion of a standard Dewar vessel which schematically illustrates the type of heat loss which I have discovered;

FIGURE 4 is a fragmentary sectional view, partially broken away, of an alternative embodiment of the invention illustrated in FIGURE 2;

FIGURE 5 is a fragmentary sectional view, partially broken away, of another alternative embodiment of the invention illustrated in FIGURE 2;

FIGURE 6 is a fragmentary sectional view, partially broken away, of yet another alternative embodiment of the invention illustrated in FIGURE 2;

FIGURE 7 is a fragmentary sectional view, partially broken away, illustrating still another embodiment of the invention illustrated in FIGURE 2;

FIGURE 8 is a fragmentary sectional view, partially broken away, of yet another alternative embodiment of the invention illustrated in FIGURE 2;

FIGURE 9 is an elevational view, partly broken away of a neck type of Dewar vessel which is used to illustrate the thermal acoustical aspects of the invention.

Referring now to the drawings a preferred embodiment of the invention will be described in connection with FIG- URE 1. A Dewar type vessel, referred to generally as 10, is comprised of an inner shell 12 and an outer shell 14 and having an evacuated space 16 therebetween. Although it is not required for the invention the evacuated space is illustrated as being substantially filled with bulk insulation 18. Any type of suitable bulk insulation might be used, but it is generally preferred that a multiple layer insulation be used. Multiple layer insulations of the type contemplated are more fully described in an article by Dr. Richard H. Kropschot of the National Bureau of Standards. This article appears in the March 1961 issue of Cryogenics, vol. 1, No. 3, and is entitled, Multiple Layer Insulation for Cryogenic Application.

A filling or liquid withdrawal tube 20- is substantially surrounded by a vacuum jacket 21 and extends from the inner container at 22, through a portion of the evacuated space 16 located at a side portion 24 of the vessel, and exits from portion 26 of the outer shell. Although not shown, the filling tube is usually terminated at a closure such as a cap or a valve and is sometimes accompanied by one or more vent tubes as will be described later.

A cyrogenic fluid 2 6 is illustrated as substantially filling the vessels storage portion 28, there being a vapor space 30 in the portion of the storage container that is not occupied by the cryogenic fiuid. A vent tube 32 enters the outer shell 14 through a suitable vacuum jacketed connection 34 and extends to the left in FIGURE 1 through the evacuated space and the bulk insulation and into the storage container at portion 34 of the inner shell. The portion of the vent tube between its leftmost end 36 and its entry point into the storage container at 34 is surrounded by a suitable vacuum jacket and is bent downwardly to form a trap 38 as shown.

As previously mentioned, more and more attention has been directed to decreasing the heat losses through the entry tubes of Dewar vessels as the quality of bulk insulation has improved. This is because the proportion of a vessels overall heat loss that is attributable to entry tubes has increased. Moreover after prolonged investigation I have noted that not only has the entry tubes proportion of the total heat loss increased, but for a given vessel even the actual amount of heat lost through entry tubes has increased as the overall storage qualities of the vessel were increased. More importantly, I have discovered that, at least in part, this increase in heat through entry tubes is caused by flow of warm fluids into the vessel through the entry tubes. In this connection it should be noted that as used herein the term entry tube includes any tube that may enter the vessel, even though it may only be used for fluid withdrawal purposes. That is, the word entry refers to the entry of heat into the vessel, not to the entry of fluids.

The warm entry tube flow is illustrated in FIGURE 3. Therein, tube 40 represents a prior art vent tube which, for purposes of illustration is shown as being inserted in the Dewar described in connection with FIGURES 1 and 2. Arrow 42 represents the fiow of cold boil-off gases from the vessels vapor space 30 to atmosphere. Arrow 4 44 represents the flow of warm fluid along the upper portion of the vent tube; and arrow 46 represents a counter of a portion of the boil-off gases. This counter flow occurs as the boil-off gases are heated during their travel from the vapor space to room temperature, for example. In the case where the stored liquid is helium, therefore, the cold boil-oft gases are warmed from about 4 K. to about 300 K. Hence, as the boil-01f gases are warmed some of the warmer portions pass to the top of the vent tube and counter flow back into the storage container.

As will now be discussed the structure described in connection with FIGURES 1 and 2 substantially obstructs both this boil-off gas counter flow and the entry of warm fluids into the vent tube. This is because the gas in the cold end 36 of the tube 32 is colder and therefore more dense. Hence, the cold gases tend to substantially fill the trap 38 and obstruct the flow of warm gas into the inner container.

An alternative structure for obstructing the flow of warm fluids into the storage container is illustrated in FIGURE 4. That is, a packing material 50 is placed in the vent tube 40 which is situated in a Dewar vessel in the same manner as the tube described in connection with FIGURE 3. Although several types of packing material may be suitable I have obtained excellent results, as will be illustrated shortly, by using a glass fiber wool which is both readily obtainable and of a low conductivity as well as being substantially inert and desirable for that reason. The presence of the packing material in the tube causes the cold gases to travel a tortuous path. Hence, the packing material tends to diifuse and mix the warm and cold gases and thus prevents convection since there is little or no temperature or density difference between the gases in the top and bottom portions of a horizontal tube.

Tests conducted in accordance with the above described method and structures have given excellent results. For example, in one instance a 500 gallon liquid nitrogen Dewar, equipped with a one and inch vent tube of the type illustrated in FIGURE 4, was compared for performance with an identically constructed standard vessel. The boil-off rate for the unmodified test vessel was 1.101 times that of the standard vessel. The test vessels vent tube was then packed with glass fiber wool for a length of about 12 inches from the warm end. When the convective currents were thusly obstructed in accordance with the method of the invention, the test vessels boil-oif rate during a 21 hour period was only 0.900 times that of the standard vessel. By using the method of the invention, therefore, the test vessels boil-01f rate was decreased by 18 percent. Moreover, because this decrease is a percentage of the vessels overall heat loss, it may be theorized that a much larger fraction of the entry tube heat loss has been substantially eliminated. Translated into dollars and cents, each one percent decrease in the boil-off rate of a 6,000 gallon trailer type helium storage vessel having a 1.5 percent per day boil-off rate represents a savings of about gallons of helium in a year. At recent market prices liquid helium costs about 20 dollars per gallon. Hence each one percent decrease represents an annular savings of about 2,000 dollars for only one such container. This is about 36,000 dollars per year where the decrease is as high as 18 percent.

In FIGURE 5 the vent tube 40 contains a spirally twisted flat bar 52 (similar to the driving rod on a childs top) which is preferably constructed of a low conductve material such as plastic, for example. The spirally twisted flat bar, in a manner similar to the packing material 50, causes the boil-off gases to travel in a spiral path as they pass out of the Dewars inner wall 12. In this manner the cold gases are mixed with the warm gases and again, because the gases in the tube are all of substantially the same density, the undesirable convective currents are substantially reduced if not eliminated. In test runs on the above described test vessel a one and inch diameter spin-ally twisted flat bar was inserted into the test vessels vent tube for a distance of 17 inches from the warm end. During a five hour period the loss rate of the thusly constructed test vessel was only 0.920 times that of the standard vessel. Hence, when the method of the invention is practiced by means of a structure that has been built as taught in connection with FIGURE 5, the overall performance of the test vessel improved by 16 percent.

From the above described test results it should be apparent that both the overall storage characteristics of a cryogenic storage vessel as well as its entry tube heat loses are greatly improved merely by obstructing the convective currents of relatively warm fluid which tend to flow inwardly from the warm ends of the vessels vent tubes. Moreover, these convective currents can be obstructed by either stutfing the vent tube with a packing material so that the boil-otf gases are forced to travel through the tortuous paths; or directing the gases through a spiral path by means of a spirally twisted flat bar; 0 rthey may be trapped out by the structure described in FIGURE 2. In this connection it will be noted that the trap also functions to inhibit warm entry flow by causing the warm gases to stratify on top of the boil-01f gases which collect in the trap portion of the entry tube.

Referring now to FIGURES 6, 7 and 8, three alternative embodiments of the structural aspects of the invention will now be described. In FIGURE 6 a rod 54 replaces the spirally twisted flat bar 52 of FIGURE 5. The rod has a helical groove 56 cut into its surface as shown. The boil-ofi gases, therefore, are directed out of the vessel in much the same manner as was described in connection with twisted bar of FIGURE 5. The structure of FIG- URE 7 is similar to that of FIGURE 2 except that the trap portion 58 of vent tube 60 is located within the evacuated space between the vessels inner and outer shells.

FIGURE 8 illustrates still another embodiment of the invention wherein a vent tube 62 contains neither a packing material nor a spiral rod nor a full trap. In this case the vent tube is merely installed at a pitch so that the lowermost portion 64 of its warm end is suitably higher than the uppermost portion of its cold end 66. This structure might be referred to as a half-trap. The device of FIGURE 8 is considerably easier to fabricate than those described in connection with FIGURES 1, 2 and 7 and is about as effective in trapping out warm gases. In this connection it should be noted that, although the trap-type embodiments of the invention have generally not been illustrated as including either a packing material or a spiral rod, these latter elements can be combined therewith as illustrated by packing material 68 stuffed into the warm end of the vent tube 62 in FIGURE 8. Similarly, it will be appreciated that although the insertion types of convection inhibitors have been illustrated as being used in horizontal vent lines, they can just as well be used in other types of entry tubes such as gage lines.

Turning now to the thermal acoustical aspects of the invention: FIGURE 9 illustrates a neck type of Dewar vessel 74) that is sometimes used to store cryogenic fluids such as liquid helium for example. As in the previously described Dewars, an inner shell 72 is suspended within an outer shell 74 by any suitable means. The space 76 between the two shells is evacuated and preferably filled with a suitable bulk insulation 77 such as the multiple layer type. A filling tube 78 extends from the inner container at 89 though a portion of the evacuated space 76 and out of the outer shell 74 M82. The inner container is partially filled with liquid helium 83 leaving a vapor space 84 at both the top of the container and inside the fill tube 78.

The lower portion of the inner container has a recess 85 for accommodating a trap portion 86 of a gage line 88. One end 90 of the gage line is connected to the helium in the inner container. The remainder of the gage line extends up one side of the vessel through the bulk insulation and is connected at its other end to one side of a suitable difierential pressure gage 92. The other side of the differential pressure gage is connected to the fill tubes vapor space by a tube 84 as shown.

The type of structure just described is comornnly used to measure the liquid level in helium Dewars, the trap portion 86 functioning merely to maintain gas in the line 88. With a thusly constructed structure, however, the end of the line 88 that is in contact with the liquid helium at 90 is colder than the end of the tube that connects with the gage 92. Hence the previously described thermal accoustical effects cause the gas within the gage line 88 to oscillate. As the gas moves towards the Warm end of the line it is compressed and therefore further heated because of the work being done on it. Moreover, as the heated gas comes into contact with a section of the gage line that is even warmer the heat from the gage line passes into the gas thereby increasing its pressure and energy content still further. When the gas oscillations are in a downwardly direction the previously heated gases give up their heat to the storage vessel. It was this type of phenomenon that Wexler was referring to when he reported an increase in the evaporation rate of a storage container by a factor of 1000.

I have discovered that the same type of fibrous packing material that acts to prevent undesirable convective currents in entry tubes also functions to substantially decrease or eliminate the heat losses in those tubes that are caused by thermal acoustical oscillations. It is for this reason that a packing material 86 is inserted in the gage line 88 of FIGURE 9.

The above described structures, therefore, not only reduce undesirable convective heat flows but also reduce heat losses caused by thermal acoustical effect. In this connection, it will be appreciated by those skilled in the art that although the effect of the packing material on reducing thermal acoustical heat losses has been illustrated in a vertical gage line it is equally applicable to any type of tube that has one end thereof closed. It is equally applicable, for example, in any of the vent tubes described in the previous figures whenever they may have the warm ends thereof closed.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in for-m and details may be made therein without departing from the spirit and scope of the invention. For example, although the method and various structural aspects of the invention have been described in relation to a relatively small Dewar, they are just as applicable to the large trailer types of Dewar vessels. In fact, it is common for large trailer types of Dewar vessels to have vent lines running from their uppermost portions down the exterior of the structure to a more readily accessible level. In this case because of the height differential between the ends of the vent lines, the previously described warm convective currents are even greater.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a Dewar type of cryogenic storage vessel, the method of reducing heat losses through an entry tube of the type which permits gases to flow between the vessels exterior and the storage portions thereof comprising the step of:

substantially obstructing convective currents of relatively warm fluid which flow from the warm end of said tube towards the cold end thereof without substantially increasing the conductive heat transfer thru said tube.

2. In a Dewar type of cryogenic storage vessel, the method of reducing heat losses through an entry tube of the type which permits gases to flow between the vessels exterior and the storage portions thereof comprising the step of:

forcing the cold boil off gases through a tortuous path as said gases flow from the cold end of said vent tube towards the warm end thereof.

3. The method of claim 2 wherein said path is a spiral.

4. In a Dewar type of cryogenic storage vessel, the method of reducing heat losses due to warm convective currents through an entry tube of the type which permits gases to flow between the vessels exterior and the storage portions thereof, comprising the step of:

mixing the warm gases attempting to enter said tube with the cold boil-off gases from said vessel.

5. The method of claim 4 wherein said mixing is brought about by forcing said gases through a tortuous path.

6. The method of claim 5 wherein said path is a spiral.

7. In a Dewar type of cryogenic storage vessel of the type in which a tube extends from the inner wall thereof through the outer wall thereof and wherein said tube is closed at its outer end, the method of reducing thermal acoustical heat losses in said tube comprising the step of:

dampening the thermal acoustical oscillations by forcing the thermal acoustically oscillated gases in said tube to oscillate through a tortuous path. 8. In a cryogenic fluid storage vessel of the type in which an inner wall defines a cryogenic fluid storage container of said vessel and wherein a space between the cold wall and a warm outer wall is evacuated, the combination comprising:

an entry tube of the type in which boil-off gases are conducted from the cryogenic fluid in the inner container through the vacuum space from said cold wall to said warm wall;

and a packing material in said entry tube for preventing warm fluid from flowing from the warm end of said entry tube to the cold end thereof.

9. The apparatus of claim 8 wherein said packing material is fibrous.

10. The apparatus of claim 9 wherein said fibrous material is comprised of glass fiber wool.

11. In a cryogenic fluid storage vessel of the type in which an inner wall defines a cryogenic fluid storage container of said vessel and wherein a space between the cold Wall and a warm outer wall is evacuated, the combination comprising:

an entry tube of the type in which boil-off gases are conducted from the cryogenic fluid in the inner container through the vacuum space from said cold wall to said warm Wall;

and a spirally twisted flat bar inserted in said entry tube.

12. In a cryogenic fluid storage vessel of the type in which an inner wall defines a cryogenic fluid storage container of said vessel and wherein a space between the cold wall and a warm outer wall is evacuated, the combination comprising:

an entry tube of the type in which boil-off gases are conducted from the cryogenic fluid in the inner container through the vacuum space from said cold wall to said warm wall;

and a rod inserted in said entry tube, said rod having a helical groove in an outer surface thereof for directing the boil-off gases in a helical path through said groove.

13. In a cryogenic fluid storage vessel of the type in which cold gas is transported substantially horizontally across an evacuated space between a cold inner wall and a warm outer wall, the combination comprising:

a tube for conducting said cold gas from said cold wall to said warm wall;

first connecting means for connecting the warm end of said tube to said warm wall;

and second connecting means for connecting the cold end of said tube to said cold wall, said second connecting means being located below said first connecting means so that said cold gas substantially fills the cold end of said tube whereby warmer gases are inhibited from entering said storage vessel through said tube.

14. The apparatus of claim 13 including a packing material in said tube.

15. The apparatus of claim 14 wherein said packing material is fibrous.

16. The apparatus of claim 15 wherein said fibrous material is comprised of glass fiber wool.

17. The apparatus of claim 13 including a spirally twisted flat bar inserted in said tube.

18. The apparatus of claim 13 including a rod inserted in said tube, said rod having a helical groove in an outer surface thereof for directing the boil-off gases in a helical path through said groove.

19. In a cryogenic storage vessel of the type in which an inner wall defines a cryogenic fluid storage container of said vessel and wherein a space between the cold wall and a warm outer wall is evacuated, the combination comprising:

an entry tube of the type in which boil-off gases are conducted from the cryogenic fluid in the inner container through the vacuum space from said cold wall to said warm wall;

and a trap located in said entry tube so that the cold fluid substantially fills the trap whereby warmer fluids are inhibited from entering said storage vessel through said entry tube.

2.0. The apparatus of claim 19 including a packing material in said entry tube.

21. The apparatus of claim 20 wherein said packing material is fibrous.

22. The apparatus of claim 21 wherein said fibrous material is comprised of a glass fiber wool.

23. The apparatus of claim 19 including a spirally twisted flat bar inserted in said entry tube.

24. The apparatus of claim 19 including a rod inserted in said entry tube, said rod having a helical groove in the surface thereof for directing the boil-off gases in a helical path through said groove.

References Cited UNITED STATES PATENTS 1,966,265 7/1934 Schmelkes 62-48 2,373,037 4/1945 Lindsay 62 2,643,953 3/1953 Sulfrian et al 220-44 X 2,691,231 10/1954 Phillips 6449 X 2,722,105 11/1955 Keyes 62 2,940,631 6/1960 Keeping 62 51X 3,142,159 7/1964 Berlad 62-43 3,201,946 8/1965 Pauliukonis 62-45 3,201,947 8/1965 Post et 'al. 62-55 3,206,939 9/1965 Wilson 62-55 FOREIGN PATENTS 924,755 5/1963 Great Britain.

LLOYD L. KING, Primary Examiner. 

1. IN A DEWAR TYPE OF CRYOGENIC STORAGE VESSEL, THE METHOD OF REDUCING HEAT LOSSES THROUGH AN ENTRY TUBE OF THE TUBE WHICH PERMITS GASES TO FLOW BETWEEN THE VESSEL''S EXTERIOR AND THE STORAGE PORTIONS THEREOF COMPRISING THE STEP OF: SUBSTANTIALLY OBSTRUCTING CONVECTIVE CURRENTS OF RELATIVELY WARM FLUID WHICH FLOW FROM THE WARM END OF SAID TUBE TOWARDS THE COLD END THEREOF WITHOUT SUBSTANTIALLY INCREASING THE CONDUCTIVE HEAT TRANSFER THRU SAID TUBE. 